The present invention concerns a spray enclosure and, more particularly, to an aerial work platform that includes a portable spray enclosure.
The surfaces of large structures such as ships, buildings, bridges, and storage tanks slowly deteriorate when exposed to the environment. Lead, zinc, tin, copper and other heavy metal based paints protect such surfaces, but must be periodically refreshed or replaced to maintain their effectiveness. During maintenance, minute particles of the coating, called overspray, are inadvertently released. If not contained, the overspray carries the heavy metals into the environment, endangering people, plants, and animals and contaminating nearby structures.
Overspray contamination is a significant and persistent problem. Shipyards often enclose the entire ship, including the workers and their equipment, within a canvas and plastic shroud to control overspray. Enshrouding a ship is expensive in material, labor, and time. Furthermore, such a total ship enclosure interferes with other necessary ship maintenance activities. Firms have developed automated machines with local overspray capture shrouds that capture the overspray. But these machines are prohibitively expensive and do not permit the operator to paint the smooth surface desired by the ship's owner. Man-sized, portable enclosures for coating application or coating cleaning have been unable to adequately capture the overspray.
In order to effectively capture the overspray, an enclosure must have a mechanism to maintain a seal in the gap between the enclosure and the surface being painted. An example of one is a man-size portable enclosure, in the form of a tube-like structure with a large opening directed towards the surface to be painted and a smaller opening on the opposite side. The painter reaches through the smaller opening and sprays paint on the surface through the larger opening. The remaining sides are the enclosure's perimeter.
Since overspray tends to follow the surface, the gap between the perimeter and the surface is where overspray escapes into the environment. A seal prevents the overspray from passing through the gap. Typically, fans withdraw air from the interior area of an enclosure. Air from outside of the enclosure flows towards the fan and through the gap. The inflow of air entrains the overspray particles and prevents their uncontrolled release into the environment. This seal can be weakened and overwhelmed for several reasons. The inflow area is the product of the perimeter length and the gap size. A large perimeter, as required for easy painter access, reduces the inflow velocity along the perimeter. Furthermore, the inflow velocity naturally varies around the perimeter. Areas closer to the fan have higher inflow velocities. Areas far from the air suction may experience no inflow.
In addition, an irregular gap increases the gap area and reduces the inflow velocity further. At various points along a large perimeter, the inflow is insufficient to keep the overspray from escaping. Furthermore, when the spray apparatus is not perpendicular to the surface, the apparatus projects paint directly at the gap, overwhelming the seal. Of course, the surfaces being painted are seldom perfectly flat, and, in fact, often tilt away from the enclosure which further affects the gap end the difficulty in sealing. Also, painters always feather the paint application by tilting the spray apparatus away from the surface to give the surface a smooth appearance and this is frequently aimed at the gap area. For these reasons man-sized portable enclosures have not successfully captured the overspray.
In U.S. Pat. No. 5,489,234 an enhanced recovery system was described that used a thin cushion of air to support a nozzle-encasing shroud. The invention's shroud impacts surface irregularities such as welding beads and limits the surface speed. While this approach was practical for paint removal, it is impractical for paint application, since the impacts ruin previously laid paint.
In U.S. Pat. No. 5,688,323 a man-sized, portable spray containment enclosure is disclosed that used a thick plastic shell and a large duct fan to pull the overspray through a filter. While this significantly increased the enclosure-surface gap, it failed to form a consistent seal and in several common situations the seal failed and released the overspray.
U.S. Pat. No. 6,171,656 discloses both a method and apparatus for collecting overspray. The apparatus included a nozzle shroud that was too heavy and bulky to be carried by a man but could be carried by a robot. This structure had a larger gap than other devices and a more consistent seal than some, but restricted the motion of the nozzle rendering the resulting paint surface unacceptable.